1. Field of the Invention
The present invention relates in general to measurement systems for media mounted on a spindle or media spooled onto a spindle. More specifically, the invention relates to storage media such as spinning magnetic hard disks, optical storage disks, tape drives and other overcoats on top of, e.g., optical filters, lenses or mirrors and especially to the positioning of magnetic hard disks mounted on a spindle. Still more specifically, the invention pertains to positioning such disks in a test system.
2. Description of Related Art
Magnetic disks are data carriers with a very large storage capacity. 30 GB can be stored on a magnetic disk of about 95 mm in diameter.
In addition to the high storage density the disks must have precise magnetic, mechanical as well as certain tribological characteristics. When later used in practice, the disks rotate at approx. 7200 revolutions per minute and more. This means that the outer edge of the disk reaches a speed of higher than 100 km/h during which the write/read element is at less than one twenty-thousandths of a millimeter (<50 nm) away from the disk surface.
The quality requirements can only be achieved through the greatest precision in manufacture and statistical process controls.
To achieve high storage densities, a thin film head is used for recording and reading data from modern high-capacity magnetic disks.
The surfaces of such disks and the surfaces of other materials like optical filters, lenses or mirrors may contain features in the sub-micrometer range, such as contamination, sputtering defects, impurities, presputter contamination, scratches, delamination defects, data defects, servo defects, etc. Especially, defects which are distributed across the surface of these thin film disks can significantly influence the production yield of disk drives. These defects can be caused by a local change in the topography or the magnetic structure. In the read back signal such defects lead to so-called “thermal asperities” or “missing pulses”.
Even sub-micrometer defects of a few nanometers height can lead to disk failure or scrap in production. The exact characterization of these defects is therefore extremely important for today's magnetic disks.
In this, it is particularly important to create a correlation between the sub-micrometer defects, e.g., the topography of the magnetic disk or its magnetic structure and the resulting disturbances in read signals or other test signals like piezoelectric response, acoustic emission response, laser doppler response, optical response, etc. This is a basis for a general analysis of defects on the storage medium.
To detect errors in the test signal, spin stand testers are used. In particular, for detecting topographic defects, glide testers or optical disk analyzers are used and for magnetic disk characterization, magnetic spin stand testers are utilized. However, the test signals of the above mentioned test systems allow only a limited interpretation of the defects based on read signal analysis, whereas other or additional test techniques allow a further investigation of defect areas or other areas of interest. Using several characterization methods leads to a complementary analysis of defects. Using a combination of these techniques permits to optimize manufacturing parameters and therefore yield to better hard disk drive (HDD) quality.
A characterization with regard to topography and the magnetic characteristics can, however, be carried out using an atomic force microscope (AFM) or magnetic force microscope (MFM). The maximum range which can be recorded with such devices is typically limited to an area of 100 μm×100 μm. However, an analysis of an area of 20 μm×20 μm or below is more convenient in terms of defect characterization precision.
A disk failure analysis is carried out in the following typical way: The storage medium having a potential defect is mounted to the spin stand tester and fixed with the spindle, e.g., by a clamp mechanism. Defects are identified at high rotational speed as a particular response of the sensor (e.g., magnetoresistive element, piezoelectric element, etc.). The position of identified defects is given precisely by a radius ri and an angular position φi in respect to reference values (φ0,r0). For example the defect angle φi is counted from a predefined reference position φ0 given by the spindle drive, spindle electronics or spindle software (e.g., a pen marker or a TTL index signal) and the alignment of the sensor against a magnetic disk or a reference caliper. ri is counted from the disk edge or a predefined written track in the magnetic storage media.
For further analysis, the storage medium is removed from the spin stand tester and inserted into the complementary analysis tool (CAT). However, the defect co-ordinate system is lost in this removal and reinsertion into the CAT. The offset induced by the removal and reinertion to the new analysis tool can be up to 700 μm, which is not suitable to perform an automated measurement, because further searches and/or individual searches are still necessary. A CAT tool is for example a SEM, SIMS, EDX, AFM or even a defect marking tool.
In order to overcome this disadvantage, a combination of a MAG tester (magnetic tester, spin tester) and an AFM has been proposed in the state of the art (U.S. Pat. No. 6,297,630) which is able to directly and quickly detect and characterize sub-micrometer defects on the surface of magnetic disks.
In Research Disclosure n428, vol. 12, 1999, page 1676, a combination of a Glide Height Tester (GHT) or Acoustic Emission Tester (AET) and an AFM is disclosed. When using a GHT, a specific measuring head flies over the rotating thin film disk. The head is not able either to read or write. However, in case it strikes an elevation on the surface of the disk, a piezoelectric signal is generated due to the hit of the head with this asperity. The test signal is correlated to the height of the elevation. The information of the height and size of defects which is determined via the GHT sometimes depends on its calibration and is sometimes not sufficient for nanometer analysis and quality control. The GHT allows a generation of a defect map (ri,φi). The proposed combination uses a similar coordinate system between the GHT and the CAT. Based on the defect co-ordinates determined by the GHT, the CATs are positioned at the defects for further analysis. Thus, the topography of the disk defects or other data can be measured directly.
As can be taken from the above, measurements on hard disks, especially on a nanoscale, are performed while the disk is located on a spindle. As a consequence that the disk is not removed from the test unit, the co-ordinate system which is defined by the spindle, the test setup and the test software is not lost.
After having generated a defect map by measuring defects on the rotating disk, each single defect now can be selected and is then specifically positioned with respect to the CAT and measured. This, however, has to be done while the disk is not rotating at, e.g., 7200 RPM, i.e., the disk being stationary or rotating only very slowly. For the positioning of defects for CAT measurement the disk and/or the CAT have to be positioned precisely relative to the measurement position. The angular position can be changed by rotating the spindle where the disk is clamped, e.g., by a stepper motor.
A specific sector of the storage medium, i.e., the magnetic disk, can be positioned at any angle at very high precision by counting encoder signals (TTL signals) in dependence of the angle relative to a zero position. This is shown in FIG. 1. These pulses may come from a commercial spindle or are generated by additional hardware or software. When the target in angular position (TAP in FIG. 1) is reached, the rotation has to be stopped by a break mechanism or the spindle rotation has to be slowed down by a defined procedure (e.g., a ramp which reduces the spindle rotation velocity). For example the stepper motor movement can be halted or can be ramped down. The ramp down of the stepper motor is associated with a ramp down time which leads to a spindle angle offset (SAO) relative to the initial angular position (TAP). This spindle offset can be compensated by a new TAPnew=TAP−SAO. However different spindle rotation velocity and stepper motor parameters might require different SAOs. SAO can be significantly high owing to the computer control time and computer communication time with the stepper motor. In particular, a hardware dependent behavior was observed. In contrast to ramp down, the abrupt stopping causes post oscillations of the spindle resulting in angular inaccuracies.
Typically, a spindle is controlled and operated by using encoder signals (typically 2) and an index signal (one index signal for one turn). These signals (e.g., TTL signals) are used to define the rotational speed and rotational direction. The number of encoder signals is well-defined for one turn. The number of encoder signals, in respect to the index signal, defines an angular position. When the spindle rotates, starting at the index signal, a counter may count only the rising edge (transition from 0V to 5V) of the TTL encoder signal. In the following, an example is given that is illustrated by FIG. 1. It is assumed that there are 1000 encoder TTL signals per rotation of the spindle. In case a sector of interest of the storage medium is to be positioned at an angle φ of 115.95 degrees relative to a zero position (for investigating a defect on the magnetic disk located at exactly this position), the spindle has to be rotated by 115.95/360×1000 encoder signals forward, in other words, by 322 steps (PIn) and a fraction of 0.83 steps (PFr).
The number of index pulses can be counted very precisely by a software or hardware counter. However, stopping a moving mass like a spindle at a certain position, e.g., in the above example, at 322 index pulses, causes a certain oscillation when it is stopped “abruptly”. It has to be mentioned that these oscillations do not lead to exactly the same end positions of the spindle in every case, depending on spindle velocity and spindle mass, spindle mass distribution and spindle friction parameters. An example of such oscillations is given in FIG. 1.
FIG. 1 shows the oscillations being represented by a continuous line. TAP is the angle position of the medium where the spindle is braked (after the 322th encoder pulse) and FA is the final angle position of the medium after the spindle has come to a complete stop. It can be seen that the counter adds 5 encoder pulses (1 to 5 in FIG. 1) during the oscillation. It has to be mentioned that in this example only TTL signals from 0 to 5V (rising edge) are counted. However, these edges from the signal appear in both rotational directions, in particular during the swing back of the spindle. Accordingly, any positioning which counts such pulses owing to oscillation of the spindle is not very exact and leads to severe errors in positioning. This, in turn, may lead to miss-positioning of defects for the CAT and as a consequence, to an analysis of an area which is not representative for the failure. In our example the miss-counting of only 1 TTL signal causes a positioning error of 50–100 μm.
Post oscillations of the spindle can be excluded by using damping parameters of the spindle in such a way that an aperiodic behavior is adjusted. This is known from literature and is used in numerous measuring instruments. However, adjusting an exact aperiodic behavior requires additional hardware for damping the spindle movement. During the spin tests at higher rotational velocities this hardware must be disabled or withdrawn. Furthermore, the damping parameters and behavior have to be well adjusted to achieve the aperiodic condition. If the aperiodic condition is not met exactly, then either the spindle swings back resulting in additional unwanted index pulses or the spindle slowly rotates towards its final position (creep-condition). Even a very small swing back can lead to an additional counting of a TTL signal. In principle, the creep-case can be used to exclude a swing-back. As explained later, the stepper motor is used for fine positioning of the spindle. In case of the creep-case-condition being adjusted and the final position of the spindle angle (i.e., the position of the defect or the like to be analyzed) being reached, the spindle rotates further slowly away from this final position. Thus, adjusting the creep condition cannot be used for fine positioning.
Both cases can be adjusted by adding a damping device (e.g., eddy current brake) which effect is described via a velocity dependent term. Its proportional factor is given by the damping factor β. Friction forces between two bodies are basically not velocity dependent and therefore do not contribute to this term. It is known from literature that a movement (e.g., a pointer in a measurement device) which is dominated by friction term leads to different end portions than a movement not dominated by the friction term. Therefore, a friction term is not wanted in measurement applications.
To solve the positioning problem, it is possible to define non-identical index pulses or a continuous signal in dependence of the angle (e.g., the height of a signal) what, however, would require more electronic effort to read out each of the pulses separately. This would lead to additional hard- and software to be installed.
In addition, it would be possible to differentiate a movement of the spindle backward or forward to separate counts. However, in this case, more soft- and hardware is required as well.
Furthermore, it would be possible to reduce the rotational positioning speed so that the above described effect of post oscillations is minimized or does not occur. This, however, causes a longer rotational positioning time which is not desired. Only a positioning time of a few seconds is tolerable for most applications.
Thus, there is a need to provide a device and a method to stop the spindle in such a way that an exact positioning of the spindle is possible, by simultaneously keeping a small overall positioning time.